A bushing is a hollow electrical insulator through which a conductor may pass. Bushings are used where high voltage lines must pass through a wall or other surface, on switchgear, transformers, circuit breakers and other high voltage equipment. A bushing is e.g. used for passing a high voltage line from an oil-filled transformer, whereby the bushing is an oil-to-air bushing with a part in oil in the transformer and a part in air outside of the transformer. Other bushings are air-to-air bushings e.g. passing high voltage lines through a wall.
A bushing may be oil-filled e.g. through a transformer tank wall, consisting of several outer parts, for instance a top piece of aluminium, an upper part of porcelain with ridges, a middle part of aluminium with a flange for fastening the bushing to the transformer tank, and a bottom part of porcelain. These different parts need to be sufficiently sealed to each other to reduce the risk of oil leaks from the oil-filled bushing (as well as the risk of moisture and dirt getting into the bushing).
Another type of oil-leakage problem is addressed in U.S. Pat. No. 6,610,933 which discloses the use of a moulded bushing assembly (e.g. epoxy-based and moulded in a one-step moulding process) for an oil-filled transformer. The document is focused on the sealing attachment of the bushing to the transformer casing to avoid oil leakage while still allowing the bushing to be detachable. The bushing itself is not oil-filled. Rather, the conductor may be integrally moulded in the bushing.